Wireless power transfer systems enable power to be transferred wirelessly from a source to a load. Inductive power transfer is a non-radiative, or near-field, type of wireless power transfer. Inductive power transfer uses an oscillating current passing through a primary coil (i.e., a transmit antenna) of a source to generate an oscillating magnetic near-field that induces currents in a secondary coil (i.e., a receive antenna) of a load. The source includes a power converter having power transistor switches which switch at controllable times to convert power of the source into the oscillating current passing through the primary coil.
Inductive power transfer is performed to wirelessly charge a load, such a traction battery of an electric vehicle, using power from the source. In such wireless electric vehicle charging systems, the transmit antenna of the source is embedded in a “charging” mat and the receive antenna (and an associated rectifier) is embedded in a designated location of the vehicle. The inductive power transfer involves inductive coupling between the antennas. For inductive power transfer to be efficient, the spacing between the antennas must be relatively close within small offset tolerances.
Inductive power transfer systems require a balance of trade-offs between resonant tuning (Q), antenna coupling, amount of coil turns of the antennas, size of the coils of the antennas, antenna coil separation offset range, and power transistor switch types that can automatically startup and operate in non-damaging, soft-switching modes of operation, given a specified load range. This balance was met by using antennas having a large physical form factor.
Specifications have aggressively reduced the allowed antenna size (i.e., the antennas are to be smaller) and increased the antenna separation offset range (i.e., the inductive power transfer system is to work with the antennas being positioned a bit farther apart). These specifications have forced resonant network tuning and operation to run near or even exceed soft-switching mode boundaries and into undesirable hard-switching modes. Hard-switching in these high current resonant networks can cause the efficiency of the inductive power transfer system to drop significantly. If the hard-switching is severe enough, then power dissipation may be increased beyond what the power transistor switches and/or system thermal design are capable of withstanding.